The device of the present invention relates to cryogenically cooled detector assemblies, and, more particularly, to miniaturized cryogenically cooled detector assemblies used for thermal imaging systems.
In most thermal imaging systems using semiconductor detection devices, it is necessary to place the detectors in a vacuum environment for two reasons: first, to protect the detectors from condensation of gases, since the detectors operate at cryogenic temperatures, and second, to minimize heat load to the cryogenic refrigerator by these same gases. In the past, the cryogenic cooler (or "refrigerator") design and general packing requirements have been such that the detector dewar/refrigerator was economically nonexpendable. Because of the refrigerator size, the packaging of the system was generally large, and, therefore, the total system costs were high. Since the package design and weight were large, the cool-down time for these systems was relatively long.
As a result, the use of cryogenically cooled infrared detectors was limited to applications in which cool-down time and portability are not critical, and in which the detector system is reusable, for example, in airborne infrared reconnaissance cameras, tank or periscope sights, etc. The cost, size, weight and cool-down time of such systems have, for the most part, barred their use in small heat-seeking munitions.
Recently, a fast cool-down, low cost "microminiature" refrigerator has become commerically available, which, if packaged properly with infrared detectors, makes possible the use of detection assemblies in a variety of small, low cost applications. These applications include infrared binoculars, munitions, and other either highly portable or expendable applications. The theory and design of the refrigerator has been fully described by Robert Wolfe and Robert Duboc, Jr., "Small Wonders: Microminiature Refrigerators for Cooling Detectors", Photonics Spectra, July, 1983. A brief summary of these devices is included here as background to the apparatus of the present invention.
Like the prior art refrigerators typically used in infrared imaging systems, the new microminiature refrigerator operates on the principle of the Joule-Thomson effect. Gas at high pressure is expanded rapidly through a small orifice and therefore cools. The cooled gas is passed through a heat exchanger to precool the high pressure incoming gas, which provides lower temperature during expansion. This regenerative process continues to the liquification temperature of the gas. In prior imaging systems, a typical refrigerator was embodied in a long cylindrical "cold finger" consisting of capillary tubes and cooling fins, wherein the cooling function was directed to the end of the cylindrical cold finger. To complete the refrigerator, a close fitting closed cylinder was required to contain and direct the cold exhaust gas over the incoming high pressure gas in the heat exchanger area. This closed cylinder of precise diameter and length is incorporated into the detector dewar. The closed end is the detector support. A vacuum container surrounds the cylinder to limit heat flow and condensation.
These prior art refrigerators have several problems which make them impractical for use in small portable or expendable systems. First, the size of the refrigerator necessarily leads to a large package (dewar) size. Second, because of the large dewar size and the mass of associated components, these devices generally require several minutes to achieve operating cryogenic temperature. Third, the cost is increased because of several factors. For example, the cost to provide the precision diameter of the closed cylinder is increased. Also, the closed cylinder is approximately two (2) inches long so that, although the detectors are small, the package must be large in order to contain the refrigerator and must be of precise size. Further, since the area of the refrigerator is large, the surrounding gas must be removed, i.e., a vacuum is required, all of which increases cost. This vacuum must be maintained over the desired life of the system so that thermal conduction through the vacuum space will not increase the refrigerator temperature. The detector sitting on the end of the cold finger must be located accurately with respect to the optical system. To achieve this precise position many parts of the detector dewar require close tolerance manufacture and special assembly jigs. Finally, obtaining a hard vacuum requires glass-to-metal seals, welds, solder joints or brazes which are relatively expensive.
In the microminiature refrigerator design, the capillary tube and expansion chamber system comprising the Joule-Thomson refrigerator is embedded in a low thermally conductive substrate, such as glass. This device may take any geometric form, but generally is comprised of thin glass plates which have been etched to provide the required ports and capillary channels for the heat exchange sections, which are laminated to form a single planar element. Conventional infrared detector systems using the microminiature refrigerator have utilized packaging concepts similar to those embodied in systems having large prior art refrigerators. Specifically, in such systems the detector assembly has been mounted on the cold spot of the microminiature refrigerator, and the refrigerator has been placed inside a large vacuuable housing with an optical window located near the detector assembly. Pump-out ports, getters, and electrical vacuum feedthroughs typical of prior art systems have been included. In some cases, the electrical leads from the detector assembly have been printed directly on the refrigerator substrate.
One detector dewar assembly using such microminiature refrigerator design is shown in U.S. Pat. No. 4,488,414, issued Dec. 18, 1984, and assigned to the same assignee as the present invention. In such design, a foam insulated heat exchanger and a back-filled gas cavity is used.
It is, accordingly, an object of the present invention to provide an improved detector packaging assembly which is compact, requires fewer parts, has no tolerance build-up and is not expensive to build.
It is another object of the present invention to provide a self-contained detector package which is suited for use in portable systems requiring "instant on" capability, for example, in infrared binoculars, or infrared gun sights.
It is a further object of the present invention to provide a refrigerator and detector assembly design which can withstand high shock environments.
It is yet another object of the present invention to provide a miniature detector/refrigerator package that will operate continuously or intermittently.